In the field of motor vehicles, it is known that the engine drives various accessories, such as an electric generator, a water pump, a fan, etc. via a belt which is driven by a main pulley mounted on a shaft of the engine. This belt circulates around pulleys respectively connected to some accessories. It is known that the tension of the belt must be adjusted at a certain level in order to efficiently drive the accessories, while keeping a satisfactory life time for the belt. This is achieved by using a belt tensioning device which is provided for keeping the tension of the belt at a certain level and also for dampening vibrations resulting from the movements of the belt.
U.S. Pat. No. 5,772,549 discloses a belt tensioning device where a conical sleeve is interposed between a receiver housing and a journal which belongs to a pulley carrier. A spring works in torsion in order to apply a torque which rotates the pulley carrier with respect to the receiver housing. This spring also works in traction in order to pull the journal against the conical sleeve. The traction effort of the spring requires that its terminal coils must be immobilized within respective recesses of the receiver housing, on the one side, and of the pulley carrier, on the other side. Thus, these two parts must have complicated shapes in order to accommodate these terminal coils, which makes them difficult to cast. Moreover, the conical sleeve is made of plastic and the quality and intensity of the damping effort depends on the geometry and on the surface roughness of the sleeves, which are difficult to precisely control.
US-A-2006/0079360 discloses a belt tensioning device where a torsion spring is used to exert a torque, whereas a strip string is used to exert a braking effort. These two springs must be coupled within the device, which is complicated, in particular with respect to proportioning their respective mechanical characteristics.
US-A-2008/0280713 discloses a belt tensioner where a series of disc springs is used to exert a braking effort, independently of the use of a main spring for generating a torque on a tensioning arm. This plurality of disc springs is expensive and they must be carefully manipulated during manufacturing of the belt tensioning device which requires their stacking. Because of the disc spring stacking, this device is bulky in an axial direction. Moreover, since the main spring must be mounted around the disc springs, the tensioning device is also bulky in a radial direction.
Similar problems might occur with chain tensioning devices which can also be used within an engine of a motor vehicle.